This invention relates to a method including thermomechanical processes for forming compacts of powdered superalloy compositions to improve resistance to time-dependant fatigue crack propagation.
It is well known that nickel based superalloys are extensively employed in high performance environments. Such alloys have been used extensively in jet engines and in gas turbines where they must retain high strength and other desirable physical properties at elevated temperatures of 540.degree. C. or more.
It is also well known that in part the desirable combination of properties of such alloys at high temperatures are at least in part due to the presence of a precipitate which has been designated as a gamma prime precipitate. More detailed characteristics of the phase chemistry of gamma prime are given in "Phase Chemistries in Precipitation Strengthening Superalloy" by E. L. Hall, Y. M. Kouh, and K. M. Chang [Proceedings of 41st. Annual Meeting of Electron Microscopy Society of America, August 1983 (p. 248)].
A problem which has been recognized with many nickel based superalloys is that they are subject to formation of cracks either in fabrication or in use, and that the cracks can initiate or propagate while under stress as during use of the alloys in such structures as gas turbines and jet engines. The propagation or enlargement of cracks can lead to part fracture or other failure.
Fatigue is a process of progressive localized permanent structural change occurring in a material subjected to fluctuating stresses and strains. It is well known that fatigue can cause failure of a material at stresses well below the stress the material is capable of withstanding under static load applications. What has been poorly understood until studies were conducted was that the formation and the propagation of cracks in structures formed from superalloys is not a monolithic phenomena in which all cracks are formed and propagated by the same mechanism, at the same rate, and according to the same criteria. The complexity of crack generation and propagation, and the interdependence of such propagation with the manner in which stress is applied is a subject on which important information has been gathered.
The period during which stress is applied to a member to develop or propagate a crack, the intensity of the stress applied, the rate of application and of removal of stress to and from the member and the schedule of this application was not well understood in the industry until a study was conducted under contract to the National Aeronautics and Space Administration. This study is reported in a technical report identified as NASA CR-165123 issued from the National Aeronautics and Space Administration in August 1980, identified as "Evaluation of the Cyclic Behavior of Aircraft Turbine Disk Alloys" Part II, Final Report, by B. A. Cowles, J. R. Warren and F. K. Hauke, and prepared for the National Aeronautics and Space Administration, NASA Lewis Research Center, Contract NAS3-21379.
A principal unique finding of the NASA sponsored study was that the rate of fatigue crack propagation was not uniform for all stresses applied nor to all manners of applying stress. More importantly, it was found that fatigue crack propagation actually varied with the frequency of the application of stress to the member where the stress was applied in a manner to enlarge the crack. More surprising still, was the finding from the NASA sponsored study that the application of stress at lower frequencies rather than at the higher frequencies previously employed in studies, actually increased the rate of crack propagation. In other words, the NASA study revealed that there was a time dependence in fatigue crack propagation. Further, the time dependence of fatigue crack propagation was found to depend not on frequency alone but on the time during which the member was held under stress for a so-called hold-time.
The most undesirable time-dependent crack-growth behavior has been found to occur when a hold time is superimposed on a sine wave variation in stress. In such a case, a test sample may be subjected to stress in a sine wave pattern, but when the sample is at maximum stress, the stress is held constant for a hold-time. When the hold-time is completed the sine wave application of stress is resumed. According to this hold-time pattern, the stress is held for a designated hold-time each time the stress reaches a maximum in following the normal sine curve. This hold-time pattern of application of stress is a separate criteria for studying crack growth. This type of hold-time pattern was used in the NASA study referred to above.
Crack growth, i.e., the crack propagation rate, in high-strength alloy bodies is known to depend upon the applied stress (.sigma.) as well as the crack length (a). These two factors are combined by fracture mechanics to form one single crack growth driving force; namely, stress intensity K, which is proportional to .sigma..sqroot.a. Under the fatigue condition, the stress intensity in a fatigue cycle represents the maximum variation of cyclic stress intensity (.DELTA.K), i.e., the difference between Kmax and K.sub.min. At moderate temperatures, crack growth is determined primarily by the Cyclic stress intensity (.DELTA.K) until the static fracture toughness K.sub.IC is reached. Crack growth rate is expressed mathematically as da/dN (.DELTA.K).sup.n. N represents the number of cycles and n is a constant which is between 2 and 4. The cyclic frequency and the shape of the waveform are the important parameters determining the crack growth rate. For a given cyclic stress intensity, a slower cyclic frequency can result in a faster crack growth rate. This undesirable time-dependent behavior of fatigue crack propagation can occur in most existing high strength superalloys.
It has been determined that at low temperatures the fatigue crack propagation rate depends essentially on the intensity at which stress is applied to components and parts of such structures in a cyclic fashion. As is partially explained above, the crack growth rate at elevated temperatures cannot be determined simply as a function of the applied cyclic stress intensity .DELTA.K. Rather, the fatigue frequency can also affect the propagation rate. The NASA study demonstrated that the slower the cyclic frequency, the faster the crack grows per unit cycle of applied stress. It has also been observed that faster crack propagation occurs when a hold time is applied during the fatigue cycle. Time dependence is a term which is applied to such cracking behavior at elevated temperatures where the fatigue frequency and hold time are significant parameters. The time dependence of fatigue crack propagation is thermally activated so that the sensitivity of time dependence can be significantly magnified at 760.degree. C. as compared to 650.degree. C.
To achieve increased engine efficiency and greater performance, constant demands are made for improvements in the strength and temperature capability of the alloys used in aircraft engines. Now, these capabilities must be coupled with low fatigue crack propagation rates and a low order of time-dependency of such rates for aircraft engine parts that are highly stressed.
Progress has been made in reducing the time dependency of fatigue crack propagation rates in nickel based superalloys U.S. Pat. No. 4,816,084 discloses a method for annealing and slow cooling superalloy compositions having a gamma prime strengthening precipitate of at least 35 percent. Test data presented in the '084 patent shows the method produces essentially time-independent fatigue crack propagation rates at 650.degree. C. The '084 patent is incorporated by reference herein.
It is known that some of the most demanding sets of properties for superalloys are those which are needed in connection with jet engine construction. Of the sets of properties which are needed, those which are needed for the moving parts of the engine are usually greater than those needed for static parts, although the sets of needed properties are different for the different components of an engine. Because some sets of properties have not been attainable in cast alloy materials, resort is sometimes had to the preparation of parts by powder metallurgy techniques.
This invention specifically relates to thermomechanical processing of superalloy compositions produced by powder metallurgy techniques and focuses on the fatigue properties. In particular the time-dependence of crack growth is addressed. Powder metallurgy refers to the fabrication of essentially fully dense stock or parts from metal powders. Fine metal powders are produced so that either each powder particle or a mixture of powders conforms to a final alloy composition. Loose powder aggregates are mechanically consolidated to form relatively dense compacts that are sintered at a temperature that causes strengthening and growth of interparticle bonds. The intrinsic strength of superalloy powders usually necessitates hot compaction in one or two steps combining the compaction and sintering operation. The method of this invention is directed towards thermomechanical processes for forming the powder compacts.
A Thermomechanical process is disclosed in U.S. Pat. No. 3,975,219 for producing an anisotropic microstructure of elongated grains that improves stress-rupture properties in nickel based superalloys having gamma prime strengthening precipitates. In the disclosed method a superalloy composition is placed in a temporary condition of superplasticity and formed by isothermal hot deformation at a specified strain rate and temperature to produce a total deformation in excess of about 10 percent. The strain rate is about 1 per minute or less and the deformation temperature is between the gamma prime solvus and 250.degree. C. below the gamma prime solvus. The deformed superalloy is progressively heated in a thermal gradient to produce the elongated grains. The hot end of the thermal gradient must exceed the gamma prime solvus temperature but cannot exceed the solidus temperature of the material.
It is an object of this invention to provide a thermomechanical process for forming compacts of powdered nickel based superalloys having at least about 35 percent gamma prime, to produce essentially time-independent fatigue crack propagation rates at elevated temperatures up to about 760.degree. C.
Another object of this invention is to form the powder compacts of superalloy compositions having a volume fraction of gamma prime greater than 35 percent, to produce an isotropic microstructure of enlarged equiaxed grains of about 50 to 60 microns in the formed compact.